Fuse



Nov. 28, 1950 D, B XTE 2,532,078

FUSE

Filed April 21, 1949 Donald llflapiarr .70? C.. on its enclosure.

Patented Nov. 28, 1950 UNITED STATES PATENT OFFICE FUSE Donald L. Baxter, Chicago, Ill., assignor to Reliable Electric Company, Chicago, 111., a corporation of Illinois Application April 21, 1949, Serial No. 88,710

1 Claim. 1

at the same time to employ a housing of ceramic material. This has been true despite the many advantages afforded by ceramic material as compared to other materials, such fiber, that have heretofore been used extensively fuse housings.

Ceramic material is particularly desirable for use ina fuse housing because of its high insulation qualities, its mechanical strengthv and freedom from warping, its non-absorbent and noninfiammable characteristics, its resistance to growth of fungus, its ease inmolding and its low cost.

Accordingly, one important object of my invention is to employ a housing of ceramic material in a fuse capable of meeting certain predetermined operating requirements.

Another and complementary object of my invention is to provide a fuse that satisfactorily meets the various operating requirements established by the Underwriters Laboratories, Inc. As is known, Underwriters Laboratories, Inc. is a non-profit organization sponsored by the National Board of Fire Underwriters for the purpose of establishing certain minimum requirements that must be met by various potentially dangerous devices before Laboratory approval given. The work of the Laboratory, if course, is aimed at reducing the number and seriousness of fire and other casualties.

The operating requirements established for fuses of the type contemplated by invention are set forth in an Underwriters Laboratories, Inc. document entitled, Subject 497, Requirements for Protectors and Arresters Intended for Signalling Circuits other than Municipal Fire Alarm Circuits, dated May 8, 1947. The re-- quirements relating to fuses prescribed in this document are quoted as follows:

13. A fuse shall be capable of carrying its rated current continuously without opening, and without causing a temperature rise of more than "15. A fuse shall. blow within five minutes when carrying 150 percent of its rated current.

17. A fuse shall remain intact and shall open the circuit without emitting sufficient fiame or molten metal to ignite surgical cotton entirely surrounding the cartridge and covering the vents, when subjected to a short circuit test on a 600- volt, direct-current circuit having a capacity of 19,1309 amperes.

My invention concerns itself solely with a fuse that is capable of meeting these three Underwriters requirements. All fuses of this general type which do not meet the above requirements are not Within the scope of my invention.

Other objects and advantages of my invention will be apparent as the description proceeds, reference being had to the accompanying drawing whereinvariousforms of the invention are shown. It is to be understood that the description and drawing are illustrative only. and are not to be taken as limiting the invention except insofar as it is limited by the claims.

In the drawing:

Fig. 1 is a view in side elevation, partly in sec tion, of a protective fuse embodying my invention;

Fig. 2 is a sectional view of line of Fig. 1;

Fig. 3 is a perspective view of one component of the invention;

Fig. l is a View in side elevation, partly in section, of a modified fuse my invention, and

Fig. 5 is a view in fragmentary section of a further modification.

The protective fuse of my invention comprises a tubular housing 5 made of ceramic material. Because of its excellent elec rical and mechanical qualities and ease of molding, it is preferred to use a steatite type ceramic.

The invention will he described in detail with particular reference to a 7 ampere fuse, the most commonly used size. It desirable to describe a fuse of a particular size in more or less physical detail because there are certain critical relationships as to sizes between various parts of the fuse. In other words, it has been found that some of the dimensions are more or less critical, and a departure from such. dimensions results in a fuse that does not meet the above listed operating requirements prescribed by Underwriters Laboratories, Inc.

Accordingly, in the case of a '7 ampere fuse, ceramic housing 5 is from four to five inches in length. The length of this housing, while not ternally between the terminal elements. is preferred to locate sleeve 20 more or less midway itself particularly critical, does bear an operating relationshi with the fuse wire, presently to be described, used within the housing.

The inside diameter and the thickness of ceramic housing 5 are not particularly critical, the primary consideration in selecting these dimensions being the physical strength of the housing. I have found that an internal diameter of /32 inch and the thickness of /64 inch are highly satisfactory. These values are set forth by way of explanation and not limitation.

A metallic terminal element 6 is mounted at each end of housing 5. In Fig. 1, left hand end, the terminal element takes the form of a sleeve '1 that has an end 8 threaded into housing 5. Sleeve i may have a flange 9 that bears against housing 5 and serves as a stop for sleeve l. A nut Ill is threaded to the other end of sleeve 1 so that a conductor (not shown) may be suitably secured to the terminal element.

A modified form of terminal element is shown at l2 in Fig. 4. Terminal element I2 is an octagonal ferrule !3 that is pressed on the end of the housing. Ferrule 13 has a reduced tubular .portion I i in axial alignment with the housing. The portion [4 is adapted to be snapped or pressed into prong-type fuse receptacles (not shown).

In both cases, Fig. 1 and Fig. 4, the metallic terminal elements are mounted on the end of housing 5 in such a manner that a substantially however, be cemented to the housing, if desired.

A substantial pure lead fuse wire l6 extends axially through housing 5 with the ends of the wire engaging the respective terminal elements at the ends of the housing. A drop of solder IT or other suitable medium is applied as shown to establish a satisfactory current carrying relation between the wire ends and the terminal elements, and to seal the otherwise open ends of the elements. Solder ll, in the case of the structure of Fig. 1, may be large enough to insure that nut ii) is not separable from the terminal element.

The diameter of the lead fuse wire it has a critical effect upon fuse operation. This diameter should approximate 0.033 inch for a fuse of the capacity here described, namely '7 amperes. Experiments have shown that if this dimension is departed from by an appreciable amount in either direction, the resultant fuse is incapable of satisfying the above-listed Underwriters requirements. Since the resistance of wire I6 and its physical strength are a function of wire diameter, the relationship between wire diameter and fuse capacity and length is apparent.

A sleeve 29 of insulating materiaI is carried on fuse wire l5 between the wire ends. Preferably, sleeve is madeof a woven glass material which may be either flexible or rigid. I have discovered that most reliable results have been obtained when the length of the sleeve is substantially less than the length of the fuse wire measured in- Also, it

between the terminal elements. It is necessary to maintain a substantial space between at least one of the sleeve ends and the adjacent terminal element to insure rupture of the wire [6 upon fuse operation.

Alternatively, sleeve 26 may take the form of a glass or other non-inflammable bead 2!, as shown in Fig. 5.

As shown, the outside diameter of sleeve is appreciably less than the inside diameter of housing 5 so that the sleeve may be supported primarily by fuse wire 16 and supported very little, if at all, by the housing.

My fuse is of the type commonly known as the compression type, and no intentional venting is employed. Thus housing 5 is imperforate as far as venting apertures are concerned.

The reasons the above described fuse meets the Underwriters requirements in a satisfactory manner, while other fuses that are different in apparently small respects fail to meet the requirements, are due to the specific construction employed. There appears to be a critical correlation between the length, diameter and material of fuse wire It, the material of ceramic housing 5 and the presence of sleeve 20.

Referring to the Underwriters requirement l3 above, it is apparent that a lead fuse wire of this diameter is capable of carrying the rated current continuously without opening and without causing a temperature rise of more than degrees C. on its enclosure.

As to satisfying the requirement [5 above, namely that the fuse shall blow within five minutes when carrying percent of its rated current, the fuse above described has been found, following extensive testing, to open in approximately a minute. However, without sleeve 26, the fuse usually takes considerably longer than five minutes to open, and in some instances, it fails to open regardless of the time allowed.

The function performed by sleeve 28 in insuring that the fuse properly opens is not understood with absolute certainty. However, it is believed that sleeve (or bead) 29 retards heat dissipation from wire l6 within the sleeve and provides a temperature node point. Thus the wire in this region reaches a higher temperature than would be the case if the sleeve were omitted. The higher temperature, of course, causes the wire to melt in this region, while the uncovered section remains solid or assumes a semi -plastic state. In addition to retarding heat dissipation, sleeve 23 constitutes a mechanical load applied to wire it. This load acts upon the melted wire and insures physical and thus electrical separation of the wire, all within a short time such as one minute when the wire is carrying 150 percent of its rated current. Greater current loads, of course,

induce a shorter rupture or fusing period as the heat generated is a product of the square of the current and the resistance of the fuse wire.

Repeated tests have demonstrated the fact that satisfactory fuse action is obtained with my ceramic housing fuse only when sleeve is used, and that unsatisfactory operation results when sleeve 29 is omitted. I am aware that certain prior fuses of this general type (although not of the ceramic housing type), such as that shown in United States Patent 550,638, have used a sleeve surrounding a fuse wire. Such sleeves were made of asbestos or other non-combustible material. The purpose of such sleeves was to quench out the flashes of flame and to prevent the discharge of particles that otherwise would occur through the vent openings when the fuse ruptured under conditions of heavy overload. As far as I know, no one prior hereto has used my above described combination of a ceramic housing, a lead fuse wire of a particular length and diameter, and a surrounding sleeve to produce a fuse that meets the above described necessary requirements.

From the above description it is thought that the construction and advantages of my invention will be readily apparent to those skilled in i the art. Various changes and modifications may be made without departing from the spirit or losing the advantages of the invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

A '7 ampere protective fuse for a communications circuit, said fuse being operable in air with air as a dielectric and being capable of carrying its rated current continuously without opening and without causing a temperature rise of more than 70 C. on the housing forming a part thereof, said fuse also being capable of opening its circuit within five minutes when carrying 150% of its rated current, and said fuse being capable of opening its circuit without emitting sufficient flame or molten metal to ignite surgical cotton entirely surrounding the cartridge when subjected to a short circuit test on a 600-volt, direct-current circuit having a capacity of 10,000 amperes, said fuse comprising: an imperforate tubular housing made of ceramic material, the length of said housing being about four or five inches, a pair of metallic terminal elements mounted one at each end of said housing, the seal between said terminal elements and said housing being substantially air-tight, substantially pure lead fuse wire extending axially through said housing and having its ends in current conducting relation with said terminal elements, the diameter of said lead fuse wire being approximately 0.033 inch, and an elongated sleeve of woven glass material loosely carried on said fuse wire between the wire ends, the sleeve length being substantially less than the length of said fuse wire measured between said terminal elements.

DONALD L. BAXTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 395,421 Cockburn Jan. 1, 1889 711,921 Cook Oct. 21, 1902 911,055 Lyng Feb. 2, 1909 1,093,893 Stoll Apr. 21, 1914 1,428,768 Fay Sept. 12, 1922 2,326,031 Hodnette et al Aug. 3, 1943 

